This invention relates to multilayered coextruded articles, and more particularly to methods of coextruding such multilayered articles to prevent layer breakup and apparatus therefor.
The coextrusion of multilayer sheets and other articles wherein individual layer thicknesses are on the order of microns is known in the art. For example, Schrenk, U.S. Pat. Nos. 3,773,882 and 3,884,606, teaches devices which prepare multilayered coextruded thermoplastic polymeric materials having substantially uniform layer thicknesses. The feedblock of the coextrusion device receives streams of the diverse thermoplastic polymeric materials from sources such as heat plastifying extruders. The streams of resinous materials are passed to a mechanical manipulating section within the feedblock. This section serves to rearrange the original streams into a multilayered stream having the number of layers desired in the final body.
Optionally, this multilayered stream may be subsequently passed through a series of layer multiplying means (sometimes termed interfacial surface generators) in order to further increase the number of layers in the final body as is described in Schrenk et al, U.S. Pat. No. 3,759,647. The multilayered stream is then passed into an extrusion die which is so constructed and arranged that streamlined flow is maintained therein. Such an extrusion device is described in Chisholm et al, U.S. Pat. No. 3,557,265. The resultant product is extruded to form a multilayered body in which each layer is generally parallel to the major surface of adjacent layers. This technology may be termed microlayer coextrusion technology because of the thinness of the layers which are formed. Microlayer coextrusion is to be distinguished from conventional multilayer coextrusion which typically involves the production of less than about fifteen layers each having thicknesses which may be from one to two orders of magnitude greater than the layer thicknesses produced in microlayer coextrusion.
One of the major problems associated with microlayer coextrusion technology has been flow instability which can occur whenever two or more polymers are simultaneously extruded through a die. Such instability may cause waviness and distortions at the polymer layer interfaces. In severe instances, the layers may become intermixed and lose their separate identities. This phenomenon, termed layer breakup, is unique to microlayer coextrusion technology where individual layer thicknesses approach about 10 .mu.m or less adjacent to the walls of the feedblock, layer multiplying means, or coextrusion die of the coextrusion apparatus.
Flow of multiple polymer layers through the feedblock and interfacial surface generators entails both shear and extensional flow. However, flow outside of the extrusion die is shear-free, extensional flow. Layer breakup occurs inside flow channels very close to the channel walls where shear flow predominates. Factors that may affect or cause layer breakup include small layer thickness (i.e., 10 .mu.m or less), shear stress, interfacial tension between polymer layers, interfacial adhesion between the polymer melt and channel walls, and combinations of these factors.
Schrenk et al, "Interfacial Flow Instability in Multilayer Coextrusion", Polymer Engineering and Science, vol. 18, no. 8 (June 1978), identified a problem of flow instability in conventional multilayer coextrusion of a three-layer sheet at the die land. The authors suggested several potential solutions including increasing skin layer thickness nearest the die wall, decreasing the viscosity of the layer nearest the die wall by either increasing temperature or switching to a lower viscosity resin, reducing the total extrusion rate, or increasing the die gap. The authors noted a further potential problem involved in the introduction of lower viscosity skin layers in that nonuniform layer distribution could occur because of the viscosity mismatch between layers.
Im et al, U.S. Pat. No. 4,540,623, teach a multilayer laminated article which includes a polycarbonate as one of the alternating layers using the apparatuses taught in the above-mentioned U.S. Pat. Nos. 3,773,882, 3,884,606, and 3,759,647. Im et al further describe the use of sacrificial or integral skin layers on the order of from about 1 to 10 mils (25.4 to 254 .mu.m) thick to increase the ease of processing the articles and to protect the surfaces thereof from damage. These exterior skin layers are added immediately prior to the multilayer film exiting from the forming die or prior to layer multiplication.
Alfrey, Jr. et al, U.S. Pat. No. 3,711,176, and Radford et al, "Reflectivity of Iridescent Coextruded Multilayered Plastic Films", Polymer and Engineering Science, Vol. 13, No. 3, pp. 216-221 (May 1973), teach a multilayered highly reflective thermoplastic body fabricated using the multilayer coextrusion devices of Schrenk, discussed above. The reflective optically thin film layers of Alfrey, Jr. et al and Radford et al relied on the constructive interference of light to produce reflected visible, ultraviolet, or infrared portions of the electromagnetic spectrum. Further, as such optically thin films are highly reflective at wavelengths where there is constructive interference, the multilayer films were made up of only a few hundred layers or less. Desired layer thicknesses could be achieved by layer thinning during extensional flow outside the feedblock and layer multiplying apparatus. Individual layer thicknesses inside the coextrusion apparatus could be maintained above the level where flow instabilities and layer breakup occur.
More recently, multilayer coextrusion technology has been used to fabricate reflective multilayer films and sheets made up of optically thick layers (optical thickness of greater than 0.45 .mu.m) or a combination of optically thick and optically very thin layers (optical thickness of less than 0.09 m). See, Wheatley et al, U.S. Pat. No. 5,122,905 and Wheatley, U.S. Pat. No. 5,122,906. The importance of uniform layers, i.e., layers having no waviness, distortions, or intermixing, is paramount in applications where the optical properties of the multilayered article are used. Even modest instabilities in processing, resulting in layer breakup in as few as 1% of the layers, may severely detract from the reflectivity and appearance of the article. Moreover, the reflectivities of multilayer films using optically thick and optically very thin layers is dependent upon the number of layer interfaces. To increase the reflectivity of the body or film, the total number of layer interfaces must be increased. As the number of layers in such articles is increased in the coextrusion apparatus, individual layer thicknesses become smaller so that the breakup of even a relatively few layers can cause substantial deterioration of the optical properties of the article even though its mechanical properties such as strength, environmental stress crack resistance, gas or moisture barrier, and heat distortion may remain largely unaffected.
To achieve reflectivities of 80% or greater, the number of layers in the bodies of Wheatley et al U.S. Pat. No. 5,122,905 and Wheatley U.S. Pat. No. 5,122,906 generally need to be in excess of one thousand or more. The creation of this number of layers requires that the individual layers be multiplied several times in interfacial surface generators. As the layers become thinner, layer instability and breakup may become a significant problem.
Accordingly, there remains a need in the art for a process and apparatus for coextruding multilayer polymeric articles which avoids layer instabilities and layer breakup problems, especially in multilayer articles having individual layer thicknesses of less than about 10 .mu.m adjacent to the walls of the feedblock, layer multiplying devices, or extrusion die of the coextrusion apparatus.